This invention relates generally to radiant beam focusing arrangements and, more particularly, to control systems in which a focusing lens is moved automatically to maintain a radiant beam accurately focused on a moving surface, such as a recording surface.
There are a number of known techniques for positioning a lens automatically in response to signals generated by a photosensitive detector indicating focus errors. For example, in one type of video playback device, a disc is rotated on a turntable, and is scanned by a light beam which is accurately focused by an objective lens onto an information track on the disc surface. A reflected beam of light returned from the information track is directed to a photocell detector, from which is derived a control signal for varying the position of the lens to maintain focus. In most cases, information signals are also derived from the same detector, as are control signals for adjusting the position of the detector radially with respect to the information track on the disc.
Typically, the information track in such a system is of the order of one micron wide. Accordingly, there is a requirement for accurately focusing the beam to a spot of about one micron in diameter on the video disc. It can be seen that slight variations in the distance between the objective lens and the disc surface can result in loss of recorded information, as well as in the introduction of undesirable frequency components into the resulting electrical signal.
The problem of focus control is even more critical in video disc recording systems. In one type of system, the information track on the disc comprises a relatively uniform sequence of pits or recesses formed in the disc surface by a laser beam. On playback, the disc surface exhibits a different optical characteristic, such as reflectivity, as a reading beam traverses it, and a cyclicly varying carrier signal can be derived from the disc during playback. The carrier signal in one system is frequency modulated with an information signal derived from a video program source, or from some other information source. Ideally, the laser beam that performs the information writing steps has to be appropriately focused and controlled in power to produce depressions and intervening lands on the disc surface such that the duty cycle associated with the recesses and lands is fifty percent. More specifically, the recesses should be so spaced and dimensioned that the signal recovered from the disc will have a value that is above its average magnitude for fifty percent of the time, and below it average for fifty percent of the time. Any other duty cycle results in the introduction of undesirable harmonic distortion of the recovered signal. Clearly, the control of the focus of the writing beam is critical in achieving this goal. Whenever the beam is out of focus, in either direction, a wider writing beam is obtained, which may not have sufficient intensity to form a recess as large as is needed to maintain a fifth percent duty cycle.
One way of controlling the focus in a recording system of this general type is to employ a separate reading laser beam directed onto the information track formed by the writing laser beam through the same focusing lens as the writing beam. The reading beam obtained from the newly formed information track is directed to a focus detector, from which will be derived an electrical signal indicative of the error in the focal position of the lens. This error signal is fed back to a lens driving motor to effect a correction of the focus position. Although satisfactory for some applications, this type of control system is not highly accurate, especially over long periods of time. Typically periodic readjustment of the control system is required, to compensate for drifts in characteristics of the electrical components used, and in laser power output. Accordingly, prior to this invention there has been a significant need for a focusing control system that does not require periodic readjustment and is more precise than control systems of the prior art. The present invention satisfies this need.